Methods of molecular cytogenetics will be used to gain further insight into the organization of specific DNA sequences in heterochromatin. The relative positions of the controlling element, ribosomal RNA cistrons, and satellite DNAs in Sciara coprophila will be mapped by in situ hybridization. Translocations of heterochromatin will be used to study if the regulation of underreplication during polytenization depends upon situation within heterochromatin, using both in situ hybridization and filter hybridization. Whether or not there are deletions or modifications of satellite DNAs in primary spermatocytes, suggesting possible functional roles for satellite DNAs in chromosome recognition and/or movement, will be determined by C-banding, in situ hybridization, gel electrophoresis of DNA fragments produced by restriction enzymes, and fingerprinting and DNA sequencing. Finally, chromosome movement on the unipolar spindle found in primary spermatocytes will be examined by electron microscopy.